Lone Wolf and Cub: Baby Cart to Hades (1972) on Netflix

Plot summary

Ogami Itto, a former shogun's executioner turned assassin for hire, drifts down a river with his small son Daigoro riding in a baby cart. A young woman on the boat drops a bundle, and Daigoro retrieves it. The shine of Itto's half-drawn sword shows bamboo reeds trailing them, so he knows Yagyu agents are following. At an inn, the same woman is to be sold into prostitution. Itto takes her place, enduring a yakuza torture of ropes, water, and canes. The yakuza leader Torizo then hires him to kill a chamberlain.